Optical filter and spectrometer

ABSTRACT

An optical filter is disclosed including two laterally variable bandpass filters stacked at a fixed distance from each other, so that the upstream filter functions as a spatial filter for the downstream filter. This happens because an oblique beam transmitted by the upstream filter is displaced laterally when impinging on the downstream filter. The lateral displacement causes a suppression of the oblique beam when transmission passbands at impinging locations of the oblique beam onto the upstream and downstream filters do not overlap.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present invention claims priority from U.S. Patent Application No.61/934,547 filed Jan. 31, 2014, which is incorporated herein byreference.

TECHNICAL FIELD

The present disclosure relates to optical components, and in particularto optical filters and spectrometers.

BACKGROUND

An optical filter is used to select a spectral band or a spectralcomponent of incoming light. A high pass filter, for example, selectslight at wavelengths longer than an edge wavelength of the filter.Conversely a low pass filter selects light at wavelengths shorter thanan edge wavelength. A bandpass filter is a distinct type of filter,which selects light at wavelengths proximate to a center wavelength ofthe filter within a bandwidth of the filter. A tunable bandpass filteris an optical filter, the center wavelength of which may be adjusted ortuned.

A spectrometer measures an optical spectrum of incoming light. Ascanning-type spectrometer may use one or more tunable bandpass filtersto select different spectral components of the incoming light. Ascanning-type spectrometer operates by scanning the center wavelength ofthe tunable bandpass filter, so as to obtain the optical spectrum.Alternatively, a polychromator-type spectrometer uses awavelength-dispersing element optically coupled to a detector array forparallel detection of the optical spectrum. However, conventionaloptical filters and spectrometers are typically large and bulky, makingit a challenge to use them in portable devices and applications.

In view of the foregoing, it may be understood that there may besignificant problems and shortcomings associated with current solutionsand technologies for optical filters and spectrometers.

SUMMARY

In accordance with the present disclosure, two or more laterallyvariable bandpass filters may be stacked at a fixed distance from eachother to reduce requirements for impinging beam collimation, or even tocompletely alleviate the need of a tapered light pipe or another lightcollimating element. When two laterally variable bandpass filters arestacked together, the upstream filter may function as a spatial filterfor the downstream filter. This happens because an oblique beamtransmitted by the upstream filter is displaced laterally when impingingon the downstream filter. The lateral displacement may result insuppression of the oblique beam, because transmission wavelengths of theupstream and downstream filters may not overlap when beam impinginglocations on the upstream and downstream filters do not overlap,resulting in suppression of oblique beams. Due to this effect, adependence of spectral selectivity of the optical filter on a degree ofcollimation of the incoming beam striking the upstream filter may belessened.

In accordance with an aspect of the disclosure, there is provided anoptical filter comprising an upstream laterally variable bandpassoptical filter and a downstream laterally variable bandpass opticalfilter. The downstream laterally variable bandpass optical filter issequentially disposed downstream of the upstream variable bandpassoptical filter and separated by a distance L along an optical path of anoptical beam. The upstream and downstream laterally variable bandpassoptical filters each have a bandpass center wavelength that graduallyvaries in a mutually coordinated fashion along a common first directiontransversal to the optical path. A dependence of spectral selectivity ofthe optical filter on a degree of collimation of the optical beam isless than a corresponding dependence of spectral selectivity of thedownstream laterally variable bandpass optical filter on the degree ofcollimation of the optical beam.

In one exemplary embodiment, the center wavelengths of the upstream anddownstream filters are monotonically e.g. linearly or non-linearlyincreasing in the first direction. The center wavelengths of theupstream and downstream filters may, but do not have to, have asubstantially identical dependence of the bandpass center wavelength onan x-coordinate along the first direction.

In accordance with the disclosure, there is further provided an opticalspectrometer comprising the above optical filter and an optical sensordisposed in the optical path downstream of the downstream laterallyvariable bandpass optical filter. The optical sensor may include aphotodetector array. The downstream laterally variable bandpass opticalfilter may be in contact with the photodetector array, for a betterspectral selectivity.

In accordance with another aspect of the disclosure, there is furtherprovided a method for obtaining a spectrum of an optical beampropagating along an optical path, the method comprising: filtering theoptical beam with an optical filter comprising an upstream laterallyvariable bandpass optical filter and downstream laterally variablebandpass optical filter, wherein the downstream laterally variablebandpass optical filter is sequentially disposed downstream of theupstream variable bandpass optical filter and separated by a distance Lalong an optical path of an optical beam, wherein the upstream anddownstream laterally variable bandpass optical filters each have abandpass center wavelength that gradually varies in a mutuallycoordinated fashion along a common first direction transversal to theoptical path, and wherein a dependence of spectral selectivity of theoptical filter on a degree of collimation of the optical beam is lessthan a corresponding dependence of spectral selectivity of thedownstream laterally variable bandpass optical filter on the degree ofcollimation of the optical beam; and detecting optical powerdistribution along the first direction downstream of the downstreamfilter.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments will now be described in conjunction with thedrawings, in which:

FIG. 1A illustrates a conventional linearly variable filter;

FIG. 1B illustrates a conventional optical spectrometer based on thelinearly variable filter of FIG. 1A;

FIG. 2A illustrates an optical filter according to the presentdisclosure, including a pair of laterally variable bandpass filters;

FIG. 2B illustrates center wavelength dependences of the laterallyvariable bandpass filters of FIG. 2A;

FIG. 2C is a side schematic view of the optical filter of FIG. 2Aillustrating a principle of spatial filtering by the optical filter;

FIG. 3 illustrates an optical filter of FIG. 2A in a sidecross-sectional view showing an acceptance angle of the optical filter;

FIGS. 4A to 4E illustrate schematic side views of various embodiments ofoptical filters of FIGS. 2A and 3;

FIGS. 5A to 5C illustrates three-dimensional views of variousembodiments of optical filters of the present disclosure;

FIG. 6A illustrates schematic cross-sectional side view of aspectrometer including optical filters of FIGS. 2A, 3, 4A to 4E, or 5Ato 5C and a photodetector array;

FIG. 6B illustrates schematic cross-sectional side view of a sealedspectrometer including optical filters of FIG. 2A, 3, 4D, or 5A to 5C;

FIGS. 7A to 7D illustrate partial cross-sectional side views of variousembodiments of the spectrometer of FIG. 6A showing mountingconfigurations of the downstream filter on the photodetector array;

FIG. 8A illustrates a plan view of a spectrometer embodiment having atilted two-dimensional (2D) detector array;

FIG. 8B illustrates optical power density distribution on different rowsof pixels of the 2D detector array of FIG. 8A;

FIG. 8C illustrates an exploded view of a multi-spectral spectrometerembodiment of the present disclosure;

FIGS. 9A and 9B illustrate three-dimensional and side views,respectively, of an optical ray-trace model of optical filters of FIGS.2A, 3, and 4B;

FIG. 10 illustrates a superimposed view of simulated optical powerdistributions of the optical ray-trace model of FIGS. 9A, 9B atdifferent numerical apertures and distances between upstream anddownstream filters;

FIGS. 11A, 11B, and 11C illustrate simulated detected optical spectra atwavelengths of 1.0 μm, 1.3 μm, and 1.6 μm, respectively;

FIG. 12 illustrates a simulated dual-line optical spectrum showing aresolving power of the simulated optical filters of FIGS. 2A, 3A-3B, and4B;

FIG. 13 illustrates a multi-wavelength spectrum of a simulatedspectrometer having the optical filter of FIG. 2A, shown in comparisonwith a multi-wavelength spectrum of a simulated spectrometer having atapered light pipe collimator and a linear variable filter;

FIG. 14 illustrates simulated spectra of a multi-wavelength lightsource, obtained with a spectrometer having the optical filter of FIG.2A at different values of the inter-filter distance L;

FIGS. 15A and 15B illustrate a plan view (FIG. 15B) of a spectrometer ofFIG. 6A;

FIG. 16 illustrates monochromatic spectra measured with the spectrometerof FIGS. 15A and 15B; and

FIG. 17 illustrates optical transmission spectra of a doped glass samplemeasured with the spectrometer of FIGS. 15A, 15B, and compared to atransmission spectrum of the doped glass sample measured with a standardMicroNIR™ spectrometer.

DETAILED DESCRIPTION

While the present teachings are described in conjunction with variousembodiments and examples, it is not intended that the present teachingsbe limited to such embodiments. On the contrary, the present teachingsencompass various alternatives and equivalents, as will be appreciatedby those of skill in the art.

As discussed above, conventional optical filters and spectrometers arelarge and bulky, which limits their applicability in portablelight-sensing devices and applications. Linearly variable filters havebeen used in spectrometers to provide wavelength separating function.Referring to FIG. 1A, a conventional linearly variable filter 10 may beilluminated with white light, which includes top 11, middle 12, andbottom 13 white light beams. The top 11, middle 12, and bottom 13 lightbeams may strike the linearly variable filter 10 at respective top 11A,middle 12A, and bottom 13A locations. The linearly variable filter 10may have a center wavelength of a passband varying linearly along anx-axis 18. For instance, the filter 10 may pass a short wavelength peak11B at the top location 11A; a middle wavelength peak 12B at the middlelocation 12A; and a long wavelength peak 13B at the bottom location 13A.

Referring to FIG. 1B with further reference to FIG. 1A, a conventionalspectrometer 19 may include the linearly variable filter 10, a taperedlight pipe 14 disposed upstream of the linearly variable filter 10, anda linear array 15 of photodetectors disposed downstream of the linearlyvariable filter 10. In operation, non-collimated incoming light 16 maybe conditioned by the light pipe 14 to produce a partially collimatedlight beam 17. The linearly variable filter 10 may transmit light atdifferent wavelengths as explained above with reference to FIG. 1A. Thetapered light pipe 14 may reduce a solid angle of the incoming light 16,thereby improving spectral selectivity of the linearly variable filter10. The linear array 15 of photodetectors may detect optical powerlevels of light at different wavelengths, thereby obtaining an opticalspectrum, not shown, of the incoming light 16.

It may therefore be desirable to reduce the size of the spectrometer 19.The tapered light pipe 14 may often be the largest element of thespectrometer 19. A collimating element, such as tapered light pipe 14,may be needed because without it, the spectral selectivity of thelinearly variable filter is degraded. This may happen because thelinearly variable filter 10 includes a stack of thin dielectric films.The wavelength-selective properties of thin film filters may begenerally dependent on the angle of incidence of incoming light, whichmay deteriorate spectral selectivity and wavelength accuracy of thinfilm filters.

Referring to FIGS. 2A and 2B, an optical filter 20 (FIG. 2A) may beprovided as described below. For example, the optical filter 20 mayinclude sequentially disposed upstream 21A and downstream 21B laterallyvariable bandpass optical filters separated by a distance L in anoptical path 22 of an optical beam 23. As shown in FIG. 2B, the upstream21A and downstream 21B filters each may have a bandpass centerwavelength λ_(T) varying in a mutually coordinated fashion along acommon first direction 25 represented by the x-axes. The first direction25 may be transverse to the optical path 22. By way of a non-limitingexample, the bandpass center wavelength λ_(T) of both the upstream 21Aand downstream 21B filters of FIG. 2A may have respective monotonic,linear dependences 24A, 24B, as shown in FIG. 2B. The center wavelengthdependences λ_(1T)(x) and λ_(2T)(x) of the upstream 21A and downstream21B filters, respectively, on the x-coordinate may be identical, orshifted with respect to each other e.g. λ_(2T)(x)=λ_(1T)(x+x₀), where isa constant; or scaled e.g. λ_(2T)(x)=cλ_(1T)(x), where c is a constante.g. 0.9<c<1.1. In other words, the term “coordinated fashion” defines apre-determined functional relationship between the center wavelengthdependences λ_(1T)(x) and λ_(2T)(x) of the upstream 21A and downstream21B filters, respectively.

The configuration of the optical filter 20 may enable a dependence ofspectral selectivity of the optical filter 20 on a degree of collimationof the optical beam 23 to be lessened as compared to a correspondingdependence of spectral selectivity of the downstream filter 21B on thedegree of collimation of the optical beam 23. This performanceimprovement of the optical filter 20 may result from a spatial filteringeffect, which may be understood by referring to FIG. 2C. Inmonochromatic light at a wavelength λ₀, the upstream 21A and downstream21B filters may be approximately represented by slits having “openings”26 corresponding to locations along the x-axes where the centerwavelength λ_(T)=λ₀. In other words, outside of the “openings” 26, theupstream 21A and downstream 21B filters may be essentially opaque forthe monochromatic light at the wavelength λ₀. The “openings” 26 definean acceptance cone, or solid angle 27 (2θ), which depends on theinter-filter distance L. Any rays outside of the solid angle 27 may beblocked, thus improving the spectral selectivity of the downstreamfilter 21B.

The operation of the optical filter 20 of FIG. 2A may be furtherexplained by referring to FIG. 3 showing the optical filter 20 in a sidecross-sectional view. In FIG. 3, the first direction 25 may behorizontal, and the center wavelength λ_(T) may increase from left toright, for both the upstream 21A and downstream 21B optical filters. Inthe example of FIG. 3, the bandpass center wavelengths λ_(T) of theupstream 21A and downstream 21B filters may be linearly dependent on thex-coordinate:

λ_(T)=λ₀ +DΔx  (1)

where λ₀ represents a reference bandpass center wavelength at areference point x₀, D represents the proportionality coefficient, termedthe “slope” of a laterally variable filter, and Δx represents an offsetfrom the reference point x₀. The slope D may correspond to the slopes ofthe linear dependences 24A and 24B in FIG. 2B, which may, but does nothave to, be identical to each other. Deviations from identical slope ofthe linear dependences 24A and 24B may be advantageous in someapplications.

In the example of FIG. 3, the upstream 21A and downstream 21B filtersmay be aligned with each other, so that the reference point x₀corresponding to the reference bandpass center wavelength λ₀ of thedownstream filter 21B is disposed directly under the reference point x₀corresponding to the reference bandpass center wavelength λ₀ of theupstream filter 21A. The upstream filter 21A may function as a spatialfilter for the downstream filter 21B, defining an angle of acceptance 30for the downstream filter 21B. The angle of acceptance 30 may be limitedby left 31L and right 31R marginal rays at the reference wavelength λ₀,each propagating at the angle θ to a normal 32 to the upstream 21A anddownstream 21B filters and striking downstream filter 21B at the samereference point x₀. The angle of acceptance 30 may be derived from apassband 33A of the upstream filter 21A as follows.

In the geometry illustrated in FIG. 3, the left marginal ray 31L maystrike the upstream filter 21A at a location x₀−Δx. Transmissionwavelength Δ_(L) at that location may be, according to Eq. (1),λ_(L)=λ₀−DΔx. Since the left marginal ray 31L is at the referencewavelength λ₀, the left marginal ray 31L may be attenuated depending onthe width of the passband 33A of the upstream filter 21A; for sake ofthis example, e.g. a 10 dB bandwidth is taken to be 2DΔx. Thus, the leftmarginal ray 31L may be attenuated by 10 dB. Similarly, the rightmarginal ray 31R may strike the upstream filter 21A at a location x₀+Δx.Transmission wavelength λ_(R) at that location may be, according to Eq.(1), λ_(R)=λ₀+DΔx. The right marginal ray 31R may also be attenuated by10 dB. All rays at the reference wavelength λ₀ within the acceptanceangle 30 may be attenuated by a value smaller than 10 dB; and all raysat the reference wavelength λ₀ outside the acceptance angle 30 may beattenuated by a value larger than 10 dB. In other words, the upstreamfilter 21A may function as spatial filter, effectively limiting thenumerical aperture (NA) of incoming light to be separated in individualwavelengths by the downstream filter 21B. This may result in reductionof the dependence of spectral selectivity of the optical filter 20 incomparison with the corresponding dependence of the spectral selectivityof the single downstream filter 21B on the degree of collimation of theoptical beam 23. In other words, if the upstream filter 21A were absentin the optical filter 20, the spectral selectivity of the optical filter20 would be much more dependent on the degree of collimation of theoptical beam 23. Typically, the optical beam 23 may result fromscattering or luminescence of a sample, not shown, so that the opticalbeam 23 is not collimated. The lack of collimation of the optical beam23 in the absence of the upstream filter 21A would result in worseningof overall spectral selectivity unless a dedicated collimating element,such as a tapered light pipe, is used. Herein, the term “spectralselectivity” may include such parameters as passband width, stray lightrejection, in-band and out-of-band blocking, etc.

For small angles θ, one may write

θ≈Δx/L  (2), or

L≈Δx/θ  (3)

When the space between the upstream 21A and downstream 21B filters isfilled with a transparent medium having a refractive index n, Eq. (3)becomes

L/n≈Δx/θ  (4)

Eq. (4) may define an approximate relationship between the inter-filterdistance L, the refractive index n of the inter-filter gap, a lateraldistance Δx along the first direction 25, related to a bandwidth of theupstream filter 21A, and the resulting acceptance half-angle θ. A moreprecise relationship may take into account the wavelength offset due tonon-zero angle of incidence, which typically results in a blue shift(i.e. towards shorter wavelength) of the bandpass center wavelengthλ_(T). For instance, the right marginal ray 31R at the referencewavelength λ₀ striking the upstream filter 21A at the position x₀+Δx maybe tilted by the angle θ, which shifts the transmission characteristicof the upstream filter 21A to shorter wavelengths. If this wavelengthdependence is to be accounted for, the shoulders of the passband 33A mayshift to the left i.e. shorter wavelengths:

λ₁≈[(λ₀ +DΔx)(n _(eff) ²−θ²)^(1/2) ]n _(eff)  (5)

where n_(eff) represents an effective refractive index of the upstreamfilter 21A.

Although in FIG. 2B, the upstream 21A and downstream 21B laterallyvariable bandpass filters have linearly variable bandpass centerwavelengths λ_(T) as defined by Eq. (1) above, the center wavelengthsλ_(T) of the upstream 21A and downstream 21B filters may bemonotonically non-linearly, e.g. parabolically or exponentially,increasing or decreasing in the first direction 25. The dependence ofthe bandpass center wavelength λ_(T) on the x-coordinate along the firstdirection 25 of the upstream 21A and downstream 21B laterally variablefilters may be identical, or may be different to enable tweaking orvarying of the acceptance angle and/or wavelength response of theoptical filter 20. In one embodiment, the bandpass center wavelengthsλ_(T) of the upstream 21A and downstream 21B filters may be aligned witheach other, such that a line connecting positions corresponding to asame bandpass center wavelength λ_(T) of the upstream 21A and downstream21B filters forms an angle of less than 45 degrees with the normal 32 tothe downstream filter 21B. For non-zero angles with the normal 32, theacceptance cone 30 may appear tilted. Thus, it may be possible to varythe acceptance cone 30 direction by offsetting the upstream 21A anddownstream 21B filters relative to each other in the first direction 25.Furthermore, the angle may vary along the first direction (x-axis) 25.

For a better overall throughput, it may be preferable to have a lateraldistance Δx₁ along the first direction 25, corresponding to a bandwidthof the upstream filter 21A larger than a corresponding lateral distanceΔx₂ along the first direction 25, corresponding to a bandwidth of thedownstream filter 21B. In one embodiment, the upstream 21A anddownstream 21B filters each may have a 3 dB passband no greater than 10%of a corresponding bandpass center wavelength λ_(T).

The upstream 21A and/or downstream 21B filters may include a thin filmlayer stack including two, three, and more different materials, e.g.,high-index and/or absorbing layers may be used to reduce overallthickness of each of the upstream 21A and downstream 21B filters.Furthermore, the upstream 21A and/or the downstream 21B filters mayinclude diffraction gratings e.g. sub-wavelength gratings, dichroicpolymers, etc.

Referring to FIG. 4A, the upstream 21A and downstream 21B filters of anoptical filter 40A may include thin film wedged interference coatings41A and 41B, deposited on respective substrates 42A and 42B joinedback-to-back. The substrates 42A and 42B may function as a transparentmedium having a refractive index n between the upstream 41A anddownstream 41B thin film wedged interference coatings. Turning to FIG.4B, a single common substrate 42 may be used in an optical filter 40B,the upstream 41A and downstream 41B thin film wedged interferencecoatings being disposed on opposite sides of the common substrate 42.The common substrate 42 may be wedged as shown in FIG. 4C, so that theupstream 41A and downstream 41B thin film wedged interference coatings(filters) of an optical filter 40C are disposed at an angle to eachother. In this case, the distance L may vary along the first direction25. The distance L variation may help one to manage spectral slopemismatch between the upstream 41A and downstream 41B filters, as well asspectral linewidth difference between the upstream 41A and downstream41B filters. To that end, the refractive index n may also vary along thefirst direction 25, at the distance L constant or varying.

FIG. 4D illustrates another configuration of an optical filter 40D, inwhich the upstream 41A and downstream 41B thin film wedged interferencecoatings may be facing each other, being disposed in a spaced apartrelationship. An optical filter 40E of FIG. 4E illustrates anotherembodiment including thin film wedged interference coatings 41A and 41Bboth facing a same direction, e.g., the optical beam 23 in this case.

Referring back to Eq. (4) with further reference to FIGS. 2A and 4A to4C, the value L/n may typically be greater than 0.2 mm. In oneembodiment, the value L/n may be less than 15 mm, e.g., between 0.2 mmand 15 mm. It should be appreciated that the distance L may correspondto a distance between the actual thin film coatings, e.g., 41A and 41Bin FIGS. 4A to 4C, and may include thicknesses of the substrates 42,42A, and/or 42B, should these substrates be in the optical path 22between the thin film coatings 41A and 41B. By way of a non-limitingillustration, in the optical filter 40B of FIG. 4B, L may represent thethickness of the substrate 42, and n may represent the refractive indexof the substrate 42.

Referring now to FIG. 5A, optical filter 50A may be similar to theoptical filter 20 of FIG. 2A, and may be similar to the optical filters40A to 40E of FIGS. 4A to 4E. The optical filter 50A of FIG. 5A,however, may further include an aperture 51A disposed in the opticalpath 22. The aperture 51A may have a width d varying in the firstdirection 25. One function of the varying width d of the aperture 51Amay be to adjust the amount of optical energy impinging on the opticalfilter 50A, which may be used to compensate for a wavelength dependenceof a magnitude of output transmission of the upstream 21A/downstream 21Bfilters, and/or a spectral response of a photodetector array (notshown).

A compensating filter may be employed for a more precise control of thefilter's spectral response and/or a spectral response of aphotodetector, not shown. Referring to FIG. 5B, optical filter 50B maybe similar to the optical filter 20 of FIG. 2A, and may be similar tothe optical filters 40A to 40E of FIGS. 4A to 4E. A spectral responseflattening filter 51B may be disposed in the optical path 22 of theoptical filter 50B for flattening a spectral response of the opticalfilter 50B. Although the spectral flattening filter 50B is shown in FIG.5B to be disposed on the upstream filter 21A, the spectral flatteningfilter 50B may be disposed on the downstream filter 21B and/or in theoptical path 22 between the upstream 21A and downstream 21B filters.

Turning now to FIG. 5C, optical filter 50C may be similar to the opticalfilter 20 of FIG. 2A, and may be similar to the optical filters 40A to40E of FIGS. 4A to 4E. The optical filter 50C of FIG. 5C, however, mayfurther include an additional filter 21C in the optical path 22. Theadditional filter 21C may have a bandpass center wavelength varying in acoordinated fashion with the bandpass center wavelengths of the upstream21A and downstream 21B filters. The additional filter 21C may alsoinclude a high pass or a low pass laterally variable filter, adispersive element such as a diffraction grating, a coating withspectrally and/or laterally variable absorption, etc. The function ofthe additional filter 21C may be to further define input numericalaperture of incoming light, and/or further improve the resolving powerof the optical filter 20. More than three laterally variable bandpassfilters 21A, 21B, . . . 21N, where N represents any integer, may be usedin the optical filter 50C.

Referring to FIG. 6A with further reference to FIG. 2A, an opticalspectrometer 60A (FIG. 6A) may include the optical filter 20 of FIG. 2Aand a photodetector array 61 disposed in the optical path 22 downstreamof the downstream filter 21B. The photodetector array 61 may have pixels62 disposed along the first direction 25 for detecting optical powerlevels of individual spectral components of the optical beam 23, e.g.,emitted by a light source 69. In a broad sense, the term “light source”may refer to a fluorescent or scattering sample, an actual light source,e.g., for absorption measurements, etc. The light beam 23 originating,e.g., from a luminescent and/or scattering sample, may generally includeconverging or diverging rays. Herein, the term “diverging” may notrequire that the rays comprising the optical beam 23 originate from asame single point. Similarly, the term “converging” may not require therays comprising the optical beam 23 to converge to a single point. Asexplained above with reference to FIGS. 2C and 3, the dual-filterstructure of the optical filter 20, including the upstream 21A anddownstream 21B bandpass laterally variable optical filters, may resultin lessening of the dependence of spectral selectivity of the opticalspectrometer 60A on a degree of collimation of the optical beam 23. Inother words, if only the downstream filter 21B were used, without theupstream filter 21A, the spectral selectivity of the opticalspectrometer may be much more dependent on the degree of collimation ofthe optical beam 23, resulting in an overall worsening of the spectralselectivity.

The photodetector array 61 may be in direct contact with the downstreamfilter 21B. The photodetector array 61 may be flooded with a pottingmaterial so as to form an encapsulation 63. One function of theencapsulation 63 may be to provide an electronic and/or thermalinsulation of the photodetector array 61, while not obscuring a clearaperture 64 of the downstream filter 21B of the optical filter 20.Another function of the encapsulation 63 may be to protect edges of theupstream 21A and downstream 21B filters from impact, moisture, etc.

Referring to FIG. 6B with further reference to FIGS. 2A and 6A, anoptical spectrometer 60B (FIG. 6B) may include the optical filter 20 ofFIG. 2A and the photodetector array 61 disposed in the optical path 22downstream of the downstream filter 21B. The optical spectrometer 60Bmay further include an enclosure 66 having a window 67 disposed in theoptical path 22 for inputting the optical beam 23. In the embodimentshown, the window 67 may include the upstream filter 21A, and theupstream 21A and downstream 21B filters are separated by a gap 65 e.g.air gap. The downstream filter 21B may be mounted directly on thephotodetector array 61. In one embodiment, a small gap, e.g., less than2 mm, may be present between the downstream filter 21B and thephotodetector array 61.

The gap 65 may allow the photodetector array 61 to be thermallydecoupled from the enclosure 66, which in its turn enables deep coolingof the photodetector array 61 by an optional thermoelectric cooler 68.The enclosure 66 may be hermetically sealed and/or filled with an inertgas for better reliability and environmental stability. A focusingelement, not shown, may be provided in the optical path 22 between thedownstream filter 21B and the photodetector array 61 for focusing theoptical beam 23 on the photodetector array 61. A sensor other than thephotodetector array 61 may be used. By way of a non-limiting example, aphotodetector may be translated relative to the optical filter 20 in thefirst direction 25.

Mounting options of the downstream filter 21B may include depositing thethin film structure of the downstream filter 21B directly on thephotodetector array 61. By way of a non-limiting example, in FIGS. 7Aand 7B, the downstream filter 21B may be deposited on a pixel side 61Aof the photodetector array 61. In some embodiments, the downstreamfilter 21B may be a wedged thin film filter, including two blockingfilter sections 71 and a bandpass filter section 72 between the twoblocking filter sections 71.

In FIG. 7B specifically, a light-absorbing mask 73 may be placed betweenthe individual pixels 62, to shield the individual pixels 62 from straylight. In FIG. 7C, an alternative mounting option is illustrated: thedownstream filter 21B may be disposed on a back side 61B of thephotodetector array 61. Of course, this mounting option may require thata substrate 61C of the photodetector array 61 be transparent to theoptical beam 23. Advantageously, the back-mounting may allow a drivercircuitry chip 74 to be flip-chip bonded to the pixel side 61A of thephotodetector array 61. Turning to FIG. 7D, the downstream filter 21Bmay be segmented by providing, e.g., etching a plurality of parallelgrooves 76, with a black filling material 75 poured into the grooves 76,the position of which may be coordinated with bars 77 of thelight-absorbing mask 73.

Referring to FIG. 8A with further reference to FIGS. 6A and 6B, aspectrometer 80A is shown in a partial plan view. The spectrometer 80Amay be similar to the spectrometers 60A of FIG. 6A and 60B of FIG. 6B.The spectrometer 80A of FIG. 8A, however, may include a two-dimensional(2D) photodetector array 88 having a plurality of individualphotodetector pixels 82. The 2D photodetector array 88 may be rotated,or clocked, by an acute angle α relative to rows 84 of the pixels 82 ofthe optical filter 20, so that upon a monochromatic illumination, aspectral line 83 is formed on the photodetector array 31 at the angle αto the rows 84 of the pixels 82 of the 2D photodetector array 88.Referring to FIG. 8B with further reference to FIG. 8A, the rotation orclocking by the angle α may cause optical power density distributions 85on different rows 84 of pixels 82 of the 2D photodetector array 88 to beoffset from each other. In this manner, instead of one spectrum, aplurality of offset spectra may be obtained, enabling a spectralresolution and wavelength accuracy increase. A signal to noise ratio mayalso be improved, e.g., by de-convoluting and averaging individualoptical power density distributions 85.

Turning now to FIG. 8C, a spectrometer 80C may be a variant of thespectrometer 80A of FIG. 8A. The spectrometer 80C of FIG. 8C may alsoinclude the 2D photodetector array 88. In FIG. 8C, the 2D photodetectorarray 88 may or may not be tilted as shown in FIG. 8A. The spectrometer80C of FIG. 8C may further include upstream 81A and downstream 81Bfilters similar to the corresponding upstream 21A and downstream 21Bfilters of the optical filter 20 of FIG. 2A, that is, having bandpasscenter wavelengths gradually varying in a mutually coordinated fashionalong the first direction 25 transversal to the optical path 22 of theoptical beam 23. In FIG. 8C, the upstream 81A and downstream 81B filterseach may include a plurality of segments 89A-1, 89A-2, 89A-3 (theupstream filter 81A) . . . and 89B-1, 89B-2, 89B-3 (the downstreamfilter 81B) arranged side by side in a second direction 87 perpendicularto the first direction 25. Each segment 89A-1, 89A-2, 89A-3 . . . of theupstream filter 81A corresponds to one of the segments 89B-1, 89B-2,89B-3 of the downstream filter 81B for operation in a dedicatedwavelength region. By way of a non-limiting example, the first pair ofsegments 89A-1 and 89B-1 may be configured for operation in thewavelength range of 1000 nm to 1200 nm, the second pair of segments89A-2 and 89B-2 may be configured for operation in the wavelength rangeof 1200 nm to 1400 nm, the third pair of segments 89A-3 and 89B-3 may beconfigured for operation in the wavelength range of 1400 nm to 1600 nm,and so on. The wavelength ranges may not need to be contiguous. Forexample, multiple segments may be provided for other wavelength regionssuch as visible wavelengths or near infrared (IR), mid IR, ultraviolet(UV), and even soft X-ray. Thus, the spectrometer 80C may be suitablefor multi-spectral sensing and/or multi-spectral imaging applications.These multi spectral sending/imaging applications may require suitablesubstrate and coating materials, as appreciated by those skilled in theart.

Referring back to FIG. 2A, a method for obtaining a spectrum of theoptical beam 23 propagating along the optical path 22 may includefiltering the optical beam 23 with optical filter 20 having upstream 21Aand downstream 21B laterally variable bandpass optical filters separatedby a distance L. As illustrated in FIG. 2B, the upstream 21A anddownstream 21B filters each may have a bandpass center wavelength λ_(T)gradually varying in a mutually coordinated fashion (e.g. 24A, 24B)along the common first direction 25 transversal to the optical path 22.Due to the sequential placement of the upstream 21A and downstream 21Bfilters, a dependence of spectral selectivity of the optical filter,such as bandwidth, out-of-band rejection, etc., on a degree ofcollimation of the optical beam 23 may be less than a correspondingdependence of spectral selectivity of the downstream filter 21B alone onthe degree of collimation of the optical beam 23.

In the next step of the method, the optical power distribution may bedetected along the first direction 25 downstream of the downstreamfilter 21B. For instance, referring back to FIGS. 6A, 6B, and 8A, thephotodetector array 61 (FIGS. 6A, 6B) or the 2D photodetector array 88(FIG. 8A) may be disposed downstream of the downstream filter 21B, andthe optical power distribution may be detected using the photodetectorarrays 61 or 88. Referring again to FIGS. 6A and 7A to 7C, thedownstream filter 21B may be disposed, e.g. deposited, directly on thephotodetector array 61, which may be flooded with a potting material soas to insulate the photodetector array 61, while not obscuring the clearaperture 64 of the downstream filter 60A.

In some embodiments, a ray-trace simulation may be performed to verifythe performance of the optical filter 20A of FIG. 2A and similar filtersof the present disclosure. Referring to FIGS. 9A and 9B, a ray-tracemodel 90 may include in sequence a Lambertian light source 99, arectangular aperture 96, an upstream laterally variable bandpass filter91A, a transparent spacer 92 having the length L, a downstream laterallyvariable bandpass filter 91B, and a photodetector 97. Input parametersof the ray-trace model 90 are summarized in Table 1 below. For example,rays 93 were traced in a sufficient number to obtain repeatable results.Each ray 93 had a pre-defined wavelength and carried a pre-definedoptical power. Optical power readings were accumulated in bins of thephotodetector 97 aligned along a dispersion direction 95, whichcorresponds to the first direction 25 in FIG. 2A. The constantparameters included the distance from the Lambertian light source 99 tothe aperture 96 of 3 mm; size of the photodetector 97 of 6.6 mm×0.25 mm;and number of bins, or pixels, of the photodetector 97 equal to 838.Varied parameters included bandwidth in % and NA in F/# of the upstream91A and downstream 91B laterally variable bandpass filters, andthickness of the transparent spacer 92. The Lambertian light source 99emitted light at eight wavelengths of 0.95 μm; 1.05 μm; 1.15 μm; 1.25μm; 1.35 μm; 1.45 μm; 1.55 μm; and 1.65 μm.

TABLE 1 Total Power power density Distance Diffuser on on to Modeldimensions diffuser diffuser Upstream Downstream L detector # (L × W mm)(W) (W/mm{circumflex over ( )}2) filter 91A filter 91B (mm) (mm) REF  3× 2.5 100.00 13.33 TLP TLP 20.0 0.07 1 10 × 1 133.33 13.33 1.4% LVF .7%LVF F/#3 1.7 0.07 F/#3 2 10 × 1 133.33 13.33 1.4% LVF .7% LVF F/#3 1.00.07 F/#3 3 10 × 1 133.33 13.33 1.4% LVF .7% LVF F/#3 1.7 0.07 F/#5 4 10× 1 133.33 13.33 1.4% LVF .7% LVF F/#3 1.0 0.07 F/#5

Referring to FIG. 10, simulation results are presented in form ofoptical power distributions accumulated in bins of the photodetector 97of the optical ray-trace model 90 of FIGS. 9A, 9B. A top graph 100corresponds to a “reference model”—a simulated commercially availableMicroNIR™ spectrometer having a tapered light pipe for lightcollimation. Plots 101 to 104 correspond to Reference models 1 to 4respectively of Table 1 above.

Turning to FIGS. 11A, 11B, and 11C, a more detailed spectral performancemay be simulated at respective wavelengths of 1.0 μm; 1.3 μm; and 1.6μm. It should be appreciated that Models 1 to 4 illustrated much betterwavelength accuracy and similar spectral selectivity. Turning to FIG.12, the resolving power of Models 1 and 3 is demonstrated using a dualspectral line at 1.3 μm, at 0.12 μm separation. It should be appreciatedthat in the results shown in FIGS. 10, 11A to 11C, and FIG. 12, Models 1to 4 did not have a tapered light pipe or another light collimatingelements, yet the Models 1 to 4 have shown an acceptable spectralbandwidth. When the tapered light pipe is excluded from the referencemodel, the spectral selectivity of the reference model becomesunacceptably low.

Table 2 below summarizes the obtained simulated performance of Models1-4.

TABLE 2 Power Peak Peak Peak Irrad. Irrad. Irrad. Resolution @ @ @ 1.0μm 1.3 μm 1.6 μm Model λ = 1.0 μm λ = 1.3 μm λ = 1.6 μm wave- wave-wave- # (W/m{circumflex over ( )}2) (W/m{circumflex over ( )}2)(W/m{circumflex over ( )}2) length length length REF 7.6 16.7 12.2 9 1117 1 2.8 9.9 15.9 8 12 15 2 4.3 14.5 21.3 9 13 15 3 5.1 11 15.9 5 9 12 48.6 17.5 25.5 7 12 13

Performance of the optical filter 60A of FIG. 6A may be verified bysimulation. Performance of a standard MicroNIR™ spectrometer containingaperture boot, tapered light pipe, InGaAs diode array, was alsosimulated to provide a reference. Turning to FIG. 13, the standardMicroNIR™ spectrometer performance may be represented by dashed-linespectrum 131 of a multi-wavelength signal between 0.9 μm and 1.7 μmseparated by 0.1 μm. Solid-line spectrum 132 illustrates the simulatedperformance of the spectrometer 60A, which is free of any collimating orlight shaping optics. Some stray light between the spectral peaks isattributed to the coating, which has not been optimized for thewavelength range used. The illumination conditions for both measurementswere identical.

Referring to FIG. 14, multi-wavelength spectra 140A-140G were obtainedby simulation using the optical filter 20 of FIG. 2A at different valuesof the inter-filter distance L ranging from 0.2 mm to 30 mm. It shouldbe appreciated that, as the inter-filter distance L increases, thefilter throughput decreases, and the out-of-band rejection of straylight 141 improves. This may happen because as the inter-filter distanceL increases, the acceptance cone 2θ of the optical filter 20 (FIGS. 2C,3) is reduced.

Turning to FIG. 15A, a spectrometer 150 may include a housing 151 havinga window 152. A optical filter 153 may include an upstream laterallyvariable filter, not shown, physically spaced at 2.08 mm from adownstream laterally variable filter, not shown. The upstream filter,not seen in FIG. 15A, may have the passband of 1.3% of the centerwavelength of 1300 nm and 900 nm to 1700 nm range. The upstream filterat the top of the optical filter 153 may have a width of 2 mm, a lengthof 8 mm, and a thickness of 1.1 mm. The downstream filter may have thepassband of 0.8% of the center wavelength of 1300 nm and 900 nm to 1700nm range. The downstream filter may have a width of 1.4 mm, a length of7.4 mm, and a thickness of 1.5 mm. A standard 128-pixel detector array,not shown, was placed 80 micrometers away from the downstream filter. Anelectronic driver 154 was used to driver the detector array.

The optical filter 153 and the electronic driver 154 may also be seen inFIG. 15B, which is a magnified view of FIG. 15A, as symbolically shownwith solid lines 155. As shown in FIG. 15B, a scale bar 156 having alength of 5 mm may be used.

Referring now to FIG. 16, emission spectrum 161 and 162 were obtainedusing the spectrometer 150 of FIGS. 15A and 15B. Emission of two lasersources at wavelengths of 1064 nm and 1551 nm was directed, in turn,onto an integrating sphere to create a lambertian illumination sourcewith a switchable emission wavelength. Integration times of thephotodetector array were adjusted, so both spectra had the same peakamplitude, because each laser had different power output levels. Noother spectral or spatial filters were used for these measurements. Theintegration sphere had a 25 mm port and was placed 35 mm away from theupstream filter. In both spectra 161 and 162, the wavelength resolutionmay be limited by the pixel structure of the photodetector array. Theinstrumental 3 dB bandwidth at 1065 nm may be estimated to be 1.2%·1065nm=12.8 nm. The instrumental 3 dB bandwidth at 1550 nm may be estimatedto be 0.82%·1550 nm=12.7 nm.

Turning to FIG. 17, transmission spectra 171 and 172 were obtained usinga NIST traceable transmission reference (in this case an Avian dopedglass reference WCT2065-025) placed in front of a halogen lamp. Thefirst spectrum 171, shown in solid line, was obtained using thespectrometer 150 of FIGS. 15A and 15B. The second spectrum 172, shown indotted line, was obtained using a standard MicroNIR1700 spectrometermanufactured by JDS Uniphase Corporation, Milpitas, Calif., USA.

In both cases, dark-state reference spectra were collected by blockingthe light source. White-state reference spectra were collected byremoving the doped glass reference from the optical path. One can seethat the first spectrum 171 is closely correlated with the secondspectrum 172. The first spectrum 171 was obtained with a 1 mm wideaperture placed in front of the spectrometer 150 of FIGS. 15A and 15B.Without the aperture, the resolution was slightly reduced, but theintegration (data collection) time decreased by a factor of three.

In the preceding specification, various embodiments have been describedwith reference to the accompanying drawings. It will, however, beevident that various modifications and changes may be made thereto, andadditional embodiments may be implemented, without departing from thebroader scope of the disclosure as set forth in the claims that follow.The specification and drawings are accordingly to be regarded in anillustrative rather than restrictive sense.

At this point it should be noted that an optical filter and spectrometerin accordance with the present disclosure as described above may involvethe processing of input data and the generation of output data to someextent. This input data processing and output data generation may beimplemented in hardware or software. For example, specific electroniccomponents may be employed in a processor, module, or similar relatedcircuitry for implementing the functions associated with providing anoptical filter and/or a spectrometer in accordance with the presentdisclosure as described above. Alternatively, one or more processorsoperating in accordance with instructions may implement the functionsassociated with the present disclosure as described above. If such isthe case, it is within the scope of the present disclosure that suchinstructions may be stored on one or more processor readable storagemedia (e.g., a magnetic disk or other storage medium), or transmitted toone or more processors via one or more signals embodied in one or morecarrier waves.

The present disclosure is not to be limited in scope by the specificembodiments described herein. Indeed, other various embodiments andmodifications, in addition to those described herein, will be apparentto those of ordinary skill in the art from the foregoing description andaccompanying drawings. Thus, such other embodiments and modificationsare intended to fall within the scope of the present disclosure.Further, although the present disclosure has been described herein inthe context of a particular implementation in a particular environmentfor a particular purpose, those of ordinary skill in the art willrecognize that its usefulness is not limited thereto and that thepresent disclosure may be beneficially implemented in any number ofenvironments for any number of purposes. Accordingly, the claims setforth below should be construed in view of the full breadth and spiritof the present disclosure as described herein.

What is claimed is:
 1. An optical filter, comprising: an upstreamlaterally variable bandpass optical filter; and a downstream laterallyvariable bandpass optical filter; wherein the downstream laterallyvariable bandpass optical filter is sequentially disposed downstream ofthe upstream variable bandpass optical filter and separated by adistance L along an optical path of an optical beam, wherein theupstream and downstream laterally variable bandpass optical filters eachhave a bandpass center wavelength that gradually varies in a mutuallycoordinated fashion along a common first direction transversal to theoptical path, and wherein a dependence of spectral selectivity of theoptical filter on a degree of collimation of the optical beam is lessthan a corresponding dependence of spectral selectivity of thedownstream laterally variable bandpass optical filter on the degree ofcollimation of the optical beam.
 2. The optical filter of claim 1,wherein the center wavelengths of the upstream and downstream filtersare monotonically increasing in the first direction.
 3. The opticalfilter of claim 2, wherein the center wavelengths of the upstream anddownstream filters are non-linearly increasing in the first direction.4. The optical filter of claim 2, wherein the center wavelengths of theupstream and downstream filters have a substantially identicaldependence of the bandpass center wavelength on an x-coordinate alongthe first direction.
 5. The optical filter of claim 2, wherein a lateraldistance Δx₁ along the first direction, corresponding to a bandwidth ofthe upstream filter, is larger than a lateral distance Δx₂ along thefirst direction, corresponding to a bandwidth of the downstream filter.6. The optical filter of claim 2, further comprising a transparentmedium having a refractive index n between the upstream and downstreamfilters.
 7. The optical filter of claim 6, wherein L/n is greater than0.2 mm.
 8. The optical filter of claim 7, wherein L/n is less than 15mm.
 9. The optical filter of claim 6, wherein L/n=Δx₁/θ wherein Δx₁ is alateral distance along the first direction, corresponding to a bandwidthof the upstream filter, and θ is an angle of acceptance of the opticalfilter.
 10. The optical filter of claim 6, wherein the medium comprisesa transparent substrate, wherein the upstream and downstream filters aredisposed on opposite sides of the substrate.
 11. The optical filter ofclaim 6, wherein the refractive index n varies along the firstdirection.
 12. The optical filter of claim 1, wherein the bandpasscenter wavelengths of the upstream and downstream filters are alignedwith each other, such that a line connecting positions corresponding toa same bandpass center wavelength of the upstream and downstream filtersforms an angle of less than 45 degrees with a normal to the downstreamfilter.
 13. The optical filter of claim 12, wherein the angle variesalong the first direction.
 14. The optical filter of claim 1, whereinthe upstream and downstream filters are disposed at an angle to eachother, so that the distance L varies along the first direction.
 15. Theoptical filter of claim 1, further comprising an aperture disposed inthe optical path, wherein the aperture has a width varying in the firstdirection.
 16. The optical filter of claim 1, further comprising aspectral response flattening filter disposed in the optical path forflattening a spectral response of the optical filter.
 17. The opticalfilter of claim 16, wherein the spectral flattening filter is disposedon the upstream or downstream filter, or therebetween.
 18. The opticalfilter of claim 1, further comprising an additional laterally variablebandpass optical filter in the optical path, the additional filterhaving a bandpass center wavelength varying in a coordinated fashionwith the bandpass center wavelengths of the upstream and downstreamfilters.
 19. The optical filter of claim 1, wherein the upstream anddownstream filters each comprise a center wavelength, and a 3 dBpassband no greater than 10% of the corresponding center wavelength. 20.The optical filter of claim 1, wherein at least one of the upstream anddownstream filters comprises thin film layers comprising three or moredifferent materials.
 21. The optical filter of claim 1, wherein at leastone of the upstream and downstream filters comprises thin film layerstack, a sub-wavelength grating, or a dichroic polymer.
 22. An opticalspectrometer, comprising: an optical filter, comprising: an upstreamlaterally variable bandpass optical filter; and a downstream laterallyvariable bandpass optical filter; wherein the downstream laterallyvariable bandpass optical filter is sequentially disposed downstream ofthe upstream variable bandpass optical filter and separated by adistance L along an optical path of an optical beam, wherein theupstream and downstream laterally variable bandpass optical filters eachhave a bandpass center wavelength that gradually varies in a mutuallycoordinated fashion along a common first direction transversal to theoptical path, and wherein a dependence of spectral selectivity of theoptical filter on a degree of collimation of the optical beam is lessthan a corresponding dependence of spectral selectivity of thedownstream laterally variable bandpass optical filter on the degree ofcollimation of the optical beam; and an optical sensor disposed in theoptical path downstream of the downstream filter.
 23. The opticalspectrometer of claim 22, further comprising a light source forproviding the optical beam, wherein the optical beam comprisesconverging or diverging rays.
 24. The optical spectrometer of claim 22,further comprising a diffuser disposed in the optical path upstream ofthe upstream filter for making intensity distribution of the opticalbeam on the upstream filter more uniform.
 25. The optical spectrometerof claim 22, wherein the optical sensor comprises a photodetector arrayhaving pixels disposed along the first direction.
 26. The opticalspectrometer of claim 25, wherein the photodetector array comprises atwo-dimensional array of pixels disposed so that when the optical beamis monochromatic, a spectral line is formed on the photodetector array,wherein the spectral line forms an acute angle with rows of thephotodetector array.
 27. The optical spectrometer of claim 25, whereinthe photodetector array comprises a two-dimensional array of pixels,wherein the upstream and downstream filters each comprise a plurality ofsegments arranged side by side in a second direction perpendicular tothe first direction, wherein each segment of the upstream filtercorresponds to a segment of the downstream filter for operation in adedicated wavelength region.
 28. The optical spectrometer of claim 25,further comprising a focusing element disposed in the optical pathbetween the downstream filter and the photodetector array, for focusingthe optical beam on the photodetector array.
 29. The opticalspectrometer of claim 25, wherein the downstream filter is in contactwith the photodetector array.
 30. The optical spectrometer of claim 22,further comprising an enclosure comprising a window disposed in theoptical path for inputting the optical beam, wherein the windowcomprises the upstream filter, and wherein the upstream and downstreamfilters are separated by a gap.
 31. A method for obtaining a spectrum ofan optical beam propagating along an optical path, the methodcomprising: filtering the optical beam with an optical filter comprisingan upstream laterally variable bandpass optical filter and downstreamlaterally variable bandpass optical filter, wherein the downstreamlaterally variable bandpass optical filter is sequentially disposeddownstream of the upstream variable bandpass optical filter andseparated by a distance L along an optical path of an optical beam,wherein the upstream and downstream laterally variable bandpass opticalfilters each have a bandpass center wavelength that gradually varies ina mutually coordinated fashion along a common first directiontransversal to the optical path, and wherein a dependence of spectralselectivity of the optical filter on a degree of collimation of theoptical beam is less than a corresponding dependence of spectralselectivity of the downstream laterally variable bandpass optical filteron the degree of collimation of the optical beam; and detecting opticalpower distribution along the first direction downstream of thedownstream laterally variable bandpass optical filter.
 32. The method ofclaim 31, wherein the optical power distribution is detected using aphotodetector array.
 33. The method of claim 32 further comprisingdepositing the downstream filter on the photodetector array.
 34. Themethod of claim 33, further comprising potting the photodetector arraywith a potting material, so as to insulate the photodetector array,while not obscuring a clear aperture of the downstream filter.